1 Corinthians 15:1-2

1 Corinthians 15:1-2. Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand. By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.

By which the apostle means, unless they had a false faith, a merely notional faith; or he may also mean "Unless what I have preached unto you should have been a fable, and therefore you will have exercised your faith upon nothing real, and so it would have been in vain."